Along with the computerization of offices in the 1980's came electronically controlled non-impact printers such as the ink jet and laser printers. Drop-on-demand inkjet printers can be piezo or thermal (bubble jet). In piezo ink jet systems, inkjetted droplets are ejected by an oscillating piezo crystal. However, the thermal ink jet dominates the drop-on-demand office ink jet market. In this system, rapid heating behind the inkjet nozzles cause a bubble of vapor to form in the fluid that is inkjetted. The resulting bubble expansion and fluid ejection from the inkjet printer cartridge causes printing to appear on the substrate.
Full-color inkjet printers are more common than color lasers and are much more economical. The main advantage of inkjet printers over lasers and other non-impact printing techniques include their low cost and simplicity. Thermal inkjet systems are capable of dispensing fluid rapidly and accurately. The technology of this and other inkjet systems are discussed in the Chemistry and Technology of Printing and Imaging Systems, edited by P. Gregory, published by Chapman & Hall, 1996. Representative thermal inkjet systems and cartridges are discussed in U.S. Pat. Nos. 4,500,895 to Buck et al., 4,513,298 to Scheu, and 4,794,409 to Cowger et al., which are all hereby incorporated by reference.
The technology of inkjet printers has undergone many changes and improvements since they first appeared. Research has been conducted to ensure that the images produced are of consistent high quality. Thus, it is important that the images be permanent by being waterfast, smearfast, smudgefast, run-fast, and the like when exposed to chemical or mechanical abrasion. Non-smearing of the image when portions of the printed page are highlighted with colored markers is of particular interest. Oftentimes, the image produced by the inkjet printer on paper is not satisfactorily fixed and smears, blurring the printed image when subjected to highlighting. This type of image is not regarded as permanent.
Image permanence is defined as transference of color from the substrate when the image printed thereon is subjected to chemical and mechanical abrasion. Highlighting is oftentimes the form of chemical and mechanical abrasion experienced. This transference of color is measured by optical density (mOD). More permanent images have lower milli-Optical Density (mOD) values.
Another desired feature of printed images is light fastness. As used herein, light fastness will mean that the images do not fade when exposed to light. Light fastness is another measure of permanence as used herein. Light fastness is measured by exposing printed images to intense light in light chambers (fadometers) and comparing print density before and after the exposure.
There have been many past attempts at improving the permanence of water-based inkjet printing systems. Included among these attempts are U.S. Pat. No. 5,549,740 to Takahashi et al., U.S. Pat. No. 5,640,187 to Kashiwakazi et al., and U.S. Pat. No. 5,792,249 to Shirota et al. which utilizes an additional or “fifth” pen to apply a colorless fluid on to the substrate. As will be seen in the comparative testing, the mOD values for the images printed thereon are quite high.
Another printing technology that is inherently more permanent than water-based inkjet are hot-melt inks. These materials are solid at room temperature and are similar to wax crayons. The colorants used in these materials are solvent dyes that are soluble in the ink vehicle or pigment dispersions. Like laser toners, these materials are incompatible with the fluids used in inkjet printing.
U.S. Pat. Nos. 5,817,169 and 5,698,017, both to Sacripante et al., disclose hot melt ink compositions which use oxazoline as a vehicle used for the colorant in a nonaqueous, hot melt inkjet ink. One of the advantages of this technology is that the waxy nature of the hot melt ink creates images that are more waterfast and may be successfully utilized on plain papers. This technology is in contrast with the instant invention, which utilizes an ordinary aqueous four-color ink pen set.
Another highly efficient printing system in common use currently is laser printers. In a laser printer or copier, light from a laser beam is used to discharge areas of a photoreceptor to create an electrostatic image of the page to be printed. The image is created by the printer controller, a dedicated computer in the printer, and is passed to the print engine. The print engine transcribes an array of dots created by the printer controller into a printed image. The print engine includes a laser scanning assembly, photoreceptor, toner hopper, developer unit, corotron, discharge lamp, fuser, paper transport, paper input feeders, and paper output trays.
The final stage of laser printing or copying is to fix toner onto the paper. Toner is very fine plastic powder that is transferred from the photoreceptor. Once transferred from the photoreceptor, it lies on the paper in a very thin coating with nothing to hold it in place. In order to fix the toner to the paper, it is heated by passing between a pair of very hot rollers, so that the plastic melts around the fibers of the paper and is “fused” into place. The image is now fixed permanently onto the paper.
The fuser of a typical laser printer is of particular interest to the printing system of this invention. In these systems, fusing or melting the polymeric resin in which the colorant is embedded converts the discrete toner particles into an amorphous film. This film becomes the permanent image that results in electrophotographic copy or laser printed copy. However, the laser printer toners are incompatible with water. Since most inkjet materials are water-based, it is not possible to use laser toners in inkjet printers, and, therefore, Inkjet technology has not yet found a way to make the printed image permanent.
U.S. Pat. No. 4,943,816 to Sporer, discloses the use of a dye-less fluid for latent imaging. The dye and ink are omitted and a colorless marking fluid is used to create a latent image to be developed in a subsequent step. Omitting the dye is believed to prolong the printhead life.
U.S. Pat. No. 4,312,268, to King et al., describes a mechanical transfer of clear or colored toner to a wet image. The mechanical transfer is not by electrostatic transfer. The powdered material adheres to the wet surfaces and the rest falls down into the housing.
U.S. Pat. No. 5,847,738, to Tutt, describes the application of a total overcoat over inkjet prints as a separate process in a sequential fashion.
Accordingly, a need remains for a printing system using water-based inkjet technology, yet, will produce permanent laser-like images. These permanent images will be consistent and stable with respect to a variety of printed substrates. An ideal situation would be combining the convenience and safety of aqueous inkjet inks with the permanence of electrophotographic copies. The present invention satisfies this in a unique manner, which is described herein.